Widely known surface modification treatments for a steel material include various heat treatments (carburization, nitriding heat treatment and induction hardening, for example) and a shot peening treatment. The surface modification treatment imparts a residual stress in the vicinity of the surface of the steel material to improve the fatigue strength of the steel material. The conditions of the surface modification treatment are determined so as to achieve a desired residual stress depending on the use of the steel material. To precisely evaluate whether the surface modification treatment has been properly performed or not, the distribution of the residual stress in the depth direction from the surface of the steel material needs to be considered.
A method of measuring the fatigue strength of a steel material is disclosed in Patent Literature 1. In Patent Literature 1, the depth at which the compressive residual stress in a steel material subjected to shot peening as the surface modification treatment reaches the peak value is evaluated. However, according to the evaluation method disclosed in Patent Literature 1, the measurement condition needs to be set for each measurement target or each condition of the surface modification treatment is changed. Thus, due to the individual difference between the materials or other variations, the evaluation cannot be performed with high precision.
Another method of measuring the fatigue strength of a steel material is disclosed in Patent Literature 2. In Patent Literature 2, the distribution of the compressive residual stress in the steel material is calculated by successively measuring the value of the output voltage of the detecting coil while successively changing the depth of permeation (magnetic permeability) of the magnetic flux in the surface of the steel material by successively changing the frequency of the exciting current passed through the exciting coil that abuts against the steel material. However, the value of the output value of the detecting coil includes the voltage component due to the variation of the magnetic permeability and the voltage component due to the impedance of the detecting coil itself. Therefore, if the characteristics of the impedance of the detecting coil itself varies due to variations of the environment (temperature, noise or the like), the reliability of the measurement value decreases. In addition, the exciting coil of this measuring apparatus needs to be designed by considering the phenomenon (lift off effect) where the detected signal varies with the distance from the steel material. However, the disclosure contains no mention with regard to this phenomenon. Thus, the measuring apparatus disclosed in Patent Literature 2 cannot accurately evaluate the compressive residual stress in the steel material.